1. Field of the Invention
Embodiments of the present invention generally relate to optimizing production of hydrocarbon wells. Particularly, embodiments of the present invention relate to an artificial lift system for moving wellbore fluids. More particularly, embodiments of the present invention relate to optimizing the production of a hydrocarbon well intermitted by a plunger lift system.
2. Description of the Related Art
The production of fluid hydrocarbons from wells involves technologies that vary depending upon the characteristics of the well. While some wells are capable of producing under naturally induced reservoir pressures, more common are wells that employ some form of an artificial lift production technique. During the life of any producing well, the natural reservoir pressure decreases as gas and liquids are removed from the formation. As the natural downhole pressure of a well decreases, the wellbore tends to fill up with liquids, such as oil and water. In a gas well, the accumulated fluids block the flow of the formation gas into the borehole and reduce the production output from the well. To combat this condition, artificial lift techniques are used to periodically remove the accumulated liquids from these wells. The artificial lift techniques may include plunger lift devices and gas lift devices.
Plunger lift production systems include the use of a small cylindrical plunger which travels through tubing extending from a location adjacent the producing formation in the borehole to surface equipment located at the open end of the borehole. In general, fluids which collect in the borehole and inhibit the flow of fluids out of the formation are collected in the tubing. Periodically, the end of the tubing located at the surface is opened via a valve, and the plunger is forced up the tubing by the accumulated reservoir pressure in the borehole. The plunger carries a load of accumulated fluids to the surface for ejection out the top of the well. After the fluids are removed, gas will flow more freely from the formation into the borehole for delivery to a gas distribution system such as a sales line at the surface. The production system is operated so that after the flow of gas from the well has again become restricted due to the further accumulation of fluid downhole, the valve is closed so that the plunger falls back down the tubing. Thereafter, the plunger is ready to lift another load of fluids to the surface upon the re-opening of the valve.
A gas lift production system is another type of artificial lift system used to increase a well's performance. The gas lift production system generally includes a valve system for controlling the injection of pressurized gas from a source external to the well, such as a compressor, into the borehole. The increased pressure from the injected gas forces accumulated formation fluid up the tubing to remove the fluids as production flow or to clear the fluids and restore the free flow of gas from the formation into the well. The gas lift system may be combined with the plunger lift system to increase efficiency and combat problems associated with liquid fall back.
The use of artificial lift systems results in the cyclical production of the well. This process, also generally termed as “intermitting,” involves cycling the system between an on-cycle and an off-cycle. During the off-cycle, the well is “shut-in” and not productive. Thus, it is desirable to maintain the well in the on-cycle for as long as possible in order to fully realize the well's production capacity.
Historically, the cyclical process of artificial lift systems is controlled by pre-selected time periods. The timing technique provides for cycling the well between on and off cycles for a predetermined period of time. Deriving the time interval of these cycles has always been difficult because production parameters considered for this task are different in every well and the parameters associated with a single well change over time. For instance, as the production parameters change, a plunger lift system operating on a short timed cycle may lead to an excessive quantity of liquids within the tubing string, a condition generally referred to as a “loading up” of the well. This condition usually occurs when the system initiates the on-cycle and attempts to raise the plunger to the surface before a sufficient pressure differential has developed. Without sufficient pressure to bring it to the surface, the plunger falls back to the bottom of the wellbore without clearing the fluid thereabove. Thereafter, the cycle starts over and more fluids collect above the plunger. By the time the system initiates the on-cycle again, too much fluid has accumulated above the plunger and the pressure in the well is no longer able to raise the plunger. This condition causes the well to shut-in and represents a failure that may be quite expensive to correct.
In contrast, a lift system that operates on a relatively long timed cycle may result in waste of production capacity. The longer cycle reduces the number of trips the plunger goes to the surface. Because well production is directly related to the plunger trips, production also decreases when the plunger trips decrease. Thus, it is desirable to allow the plunger to remain at the bottom only long enough to develop a sufficient pressure differential to raise the plunger to the surface.
Improvements to the timing technique include changing the predetermined time period in response to the well's performance. For example, U.S. Pat. No. 4,921,048, incorporated herein by reference, discloses providing an electronic controller which detects the arrival of a plunger at the well head and monitors the time required for the plunger to make each particular round trip to the surface. The controller periodically changes the time during which the well is shut in to maximize production from the well. Similarly, in U.S. Pat. No. 5,146,991, incorporated herein by reference, the speed at which the plunger arrives at the well head is monitored. Based on the speed detected, changes may be made to the off-cycle time to optimize well production.
The forgoing arrangements, while representing an improvement in operating plunger lift wells, still fail to take into account some variables that change during the operation of a well. For example, sales lines pressure fluctuations affect the optimal time to commence the on cycle. A fluctuating sales line pressure will cause a change in the effective pressure available to lift liquid out of the well. Simple self-adjusting timed cycle does not take this variable into account when adjusting the length of the cycle.
There is a need, therefore, for an improved well control apparatus and method that monitor and adjust well operations to improve well production. There is also a need for a controller that optimizes the plunger lift cycle to improve the efficiency of the production from the well.